The present invention relates to a microwave switch matrix and more particularly to a switch matrix which can be fabricated utilizing microwave monolithic integrated circuit (MMIC) techniques as those employing gallium arsenide.
RF switch matrices are employed in many applications. Such matrices are used in satellite communications for spot beam transmission. The prior art employed hybrid switch matrices which were made and fabricated from passive microwave components, such as the use of PIN diodes, ferrites, as well as non-planar structures. Switch matrices require high isolation between two non-connected ports. In any event, the hybrid switch matrices which use a non-planar structure to obtain high isolation are extremely large and such circuits cannot be conventionally fabricated employing microwave monolithic integrated circuit techniques (MMIC). Such switch matrices may also be employed in conjunction with phased array radar or phased array antenna systems.
Modern technology has extensively employed monolithic microwave integrated circuits (MMIC) which circuits are fabricated on gallium arsenide substrates. These circuits offer the potential for significant reductions in size, weight and cost, together with increased reliability over conventional hybrid microwave components. Such circuits are widely employed and are particularly attractive in applications as for example in satellite antenna beam switching applications, as well as in applications involving phased array antenna systems such as those employed for example in radar systems. Such antennas normally consist of an assembly of closely packed radiator elements, each of which is driven by a microwave transmit/receive (T/R) circuit module. In such a phased array radar the radiating elements are fed with varying amplitude and phases to shape and position the beam. As such, these systems employ many components such as switch matrices, phase shifters, attenuators, switches, power dividers and so on. As one can understand, in order to optimumly employ such systems and to utilize such systems with minimum cost, MMIC technology is employed.
For an example of suitable circuits which employ such technology, reference is made to co-pending application entitled "A Monolithic Active Digital Attenuator/Amplifier Apparatus" filed on Sep. 22, 1988 as Ser. No. 07/247,966, now abandoned, for T. H. Chen and M. Kumar, the inventors herein, and assigned to the Siemens Corporation, the assignee herein. Reference is also made to a co-pending application entitled "Digitally Controlled Variable Gain Power Divider Apparatus Particularly Adapted for Microwave Applications" filed on Sep. 22, 1988 as Ser. No. 07/247,979, now abandoned, for T. H. Chen and M. Kumar, the inventors herein, and assigned to the same assignee.
In the above-noted applications there are shown circuit arrangements for providing attenuation as well as power division which employ MESFET devices which devices and associated structures can be conveniently integrated utilizing MMIC technology.
The prior art was aware of the need for switch matrices and in particular for such switch matrices which could be fabricated employing microwave integrated circuit techniques. For an example of such prior art, reference is made to U.S. Pat. No. 4,731,594 issued on Mar. 15, 1988 entitled "Planar Active Component Microwave Switch Matrix And Air Bridge For Use Therewith" to M. Kumar and assigned to the General Electric Company. In that patent there is shown a switch matrix for selectively connecting various inputs to various outputs. The switch matrix, as shown in that patent, includes a semi-insulative substrate on one side of which are conductors arranged in rows and columns. The interconnection of the rows and columns form the intersection of the matrix. The patent further shows the use of air bridges which separate the row and column conductors in order to provide isolation. The structure employs active power dividers as well as active power combiners. The power dividers and combiners employ double dual gate FET devices. These devices incorporate two dual gate FETs which share a common source electrode. The patent describes the fabrication of such devices utilizing gallium arsenide integrated circuit techniques in order to obtain a switching matrix configuration.
As one can ascertain from the above-noted patent, there is required a plurality of switches which switches are positioned between an active power divider and an active power combiner and which switches are necessary for circuit operation. Thus the above-noted circuit, apart from the particular devices employed, requires additional components in terms of these switches which make the operation of the circuit difficult and further requires an increased number of components in order to implement operation.
Reference is also made to U.S. Pat. No. 4,472,691 which issued on Sep. 18, 1984 entitled "Power Divider/Combiner Circuit As For Use In A Switching Matrix" issued to M. Kumar, et al. and assigned to The RCA Corporation. This patent describes a 1 port to N port passive signal power divider which for example can be employed in conjunction with switch matrices.
Reference is also made to U.S. Pat. No. 4,609,889 issued on Sep. 2, 1986 entitled "Microwave Frequency Power Combiner" to M. Kumar and assigned to The RCA Corporation. This patent essentially shows an in-phase power combiner constructed of a double field effect transistor having a common drain, first and second gates and first and second source electrodes.
Reference is also made to U.S. Pat. No. 4,611,184 entitled "Microwave Frequency Power Divider" issued on Sep. 9, 1986 to M. Kumar and assigned to the RCA Corporation. This patent shows an in-phase power divider which includes a double field effect transistor having a common source, first and second gates electrodes and first and second drains.
As one can ascertain from the above, the use of switching matrices, including field effect devices for implementing active power dividers and power combiners, are disclosed in the prior art. As indicated, such devices, while utilizing active power dividers and combiners necessitate the use of additional switches to implement matrix operation.
As one can ascertain from the above-referenced patents, monolithic circuits are usually designed on a thin, semi-insulating gallium arsenide substrate. Essentially, components for such a monolithic circuit may be formed by a deposition method, such as a liquid phase epitaxy, vapor phase epitaxy, ion implantation, evaporation, or sputtering. In this manner the circuit elements that can be formed on monolithic circuit substrates include transmission lines, lumped overlay or interdigital capacitors, lumped spiral inductors, thin film resistors, FETs and, when absolutely necessary, transmission line stubs.
Since space is at a premium, lumped elements are preferred to distributed elements The preferred transmission lines are microstrip and coplanar waveguide. Of these two, microstrip has less loss and can be made with characteristic impedances as high as 90 ohms on 125 um gallium arsenide substrates. Since the complete circuit is manufactured using automated techniques, no final adjustments can be made on it after it has been produced. The circuit design must rely heavily on computer modeling, optimization and redundant component configurations.
In such monolithic circuits, FET devices are relatively easy to make. Thus FETs are used for a wide variety of purposes including amplifiers, mixers, frequency multipliers and resistors. Such monolithic circuits are very likely to find increased use in microwave applications requiring large numbers of compact units. The prior art is aware of such considerations and such uses for such circuits. See for example a paper entitled "GaAs IC Applications in Electronic Warfare, Radar and Communication Systems" by D. G. Fisher, published in the Microwave Journal, May 1988, pp. 275-292. Also see an article entitled "Broadband MMIC's For System Applications" by R. W. Bierig et al., published in the Microwave Journal, May 1988, pp. 251-270.
Hence, there is a widespread use of MMIC technology which conventionally employs GaAs substrates to fabricate suitable microwave devices. Such prior art devices include metal semiconductor field effect transistors (MESFETs) fabricated on GaAs substrates. These transistors, as indicated, have been widely employed in the microwave art for frequencies above 5 GHz and have been utilized where improved gain and noise figure are required. The characteristics of such devices have been described in the co-pending applications as well as in some of the cited references. Techniques for fabricating such devices, including techniques for fabricating the transmission lines associated with such devices, are known in the prior art.
It is an object of the present invention to provide an improved monolithic active switch matrix which employs an active power divider switch, an active power combiner switch and an air bridge for use therewith and to provide isolation.
Essentially, a main object of the present invention is to provide a digitally controlled monolithic active switch matrix which matrix employs integratable components and which components can be employed to fabricate large switch arrays utilizing the same basic circuit configurations.
The present invention employs structure which reduces the number of components necessary to provide an active switch matrix as compared to prior art techniques.